Organic electroluminescent device

ABSTRACT

This invention provides an organic electroluminescent element (organic EL element) utilizing phosphorescence which emits light efficiently with high luminance at low current density, shows good driving stability and is applicable to display devices such as flat panel displays and illuminating devices. The element comprises an anode, organic layers and a cathode piled one upon another on a substrate, at least one of the organic layers is a light-emitting layer containing a host material and a dopant material and a pyrazole-derived compound having 2-4 pyrazole structures represented by the following formula I in the same molecule is used as said host material;  
                 
 
wherein, Ar 1 -Ar 3  are independently hydrogen or substituted or unsubstituted aromatic hydrocarbon groups and at least one of Ar 1 -Ar 3  is a group other than hydrogen.

FIELD OF TECHNOLOGY

This invention relates to an organic electroluminescent element and,more particularly, to a thin-film device which emits light when anelectrical field is applied to its organic light-emitting layer.

BACKGROUND TECHNOLOGY

In the development of electroluminescent elements utilizing organicmaterials (hereinafter referred to as organic EL element), the kind ofelectrodes was optimized for the purpose of improving theelectron-injecting efficiency from the electrode and an element in whicha hole-transporting layer of an aromatic diamine and a light-emittinglayer of 8-hydroxyquinoline aluminum complex are disposed as thin filmsbetween the electrodes has been developed (Appl. Phys. Lett., Vol. 51,p. 913, 1987) to bring about a noticeable improvement in luminousefficiency over the conventional elements utilizing single crystals ofanthracene and the like. Following this, the developmental works oforganic EL elements have been focused on their commercial applicationsto high-performance flat panels characterized by self luminescence andhigh-speed response.

In order to improve the efficiency of such organic EL elements stillfurther, various modifications of the aforementioned basic structure ofanode/hole-transporting layer/light-emitting layer/cathode have beentried by suitably adding a hole-injecting layer, an electron-injectinglayer and an electron-transporting layer. For example, the followingstructures are known: anode/hole-injecting layer/hole-transportinglayer/light-emitting layer/cathode; anode/hole-injectinglayer/light-emitting layer/electron-transporting layer/cathode; andanode/hole-injecting layer/light-emitting layer/electron-transportinglayer/electron-injecting layer/cathode. The hole-transporting layer hasa function of transporting the holes injected from the hole-injectinglayer to the light-emitting layer while the electron-transporting layerhas a function of transporting the electrons injected from the cathodeto the light-emitting layer.

A large number of organic materials conforming to the function of theselayered structures have been developed.

The aforementioned element comprising the hole-transporting layer of anaromatic diamine and the light-emitting layer of 8-hydroxyquinolinealuminum complex and many other elements utilize fluorescence. Now, theutilization of phosphorescence, that is, emission of light from thetriplet excited state, is expected to enhance the luminous efficiencyapproximately three times that of the conventional elements utilizingfluorescence (singlet). To achieve this object, studies have beenconducted on the use of coumarin derivatives and benzophenonederivatives in the light-emitting layer, but the result was nothing butextremely low luminance. Thereafter, the use of europium complexes wasattempted, but it was unable to obtain high luminous efficiency.

The prior technical documents relating to this invention are listedbelow.

-   -   Patent literature 1: JP2002-352957 A    -   Patent literature 2: JP2001-230079 A    -   Patent literature 3: JP2001-313178 A    -   Patent literature 4: JP2003-45611A    -   Patent literature 5: JP2002-158091A    -   Non-patent literature 1: Nature, Vol. 395, p. 151, 1998    -   Non-patent literature 2: Appl. Phys. Lett., Vol. 75, p. 4, 1999

The possibility of emitting red light at high efficiency by the use of aplatinum complex (PtOEP) is reported in the aforementioned non-patentliterature 1. Thereafter, it is reported in non-patent literature 2 thatthe efficiency of emitting green light has been improved markedly bydoping the light-emitting layer with iridium complexes (Ir(ppy)3). It isreported further that optimization of the light-emitting layer enablesthese iridium complexes to show extremely high luminous efficiency evenwhen the structure of an element is simplified.

In applying organic EL elements to display devices such as flat paneldisplays, it is necessary to improve the luminous efficiency and at thesame time to secure the driving stability. The organic EL elementsutilizing phosphorescent molecules of Ir(ppy)3 described in non-patentliterature 2, although highly efficient, are not suitable for practicaluse because of their insufficient driving stability at the present time(Jpn. J. Appl. Phys., Vol. 38, L1502, 1999).

The main cause of the deterioration of the aforementioned drivingstability is presumed to be the deterioration of the shape of thin filmof the light-emitting layer in the structure of an element such assubstrate/anode/hole-transporting layer/light-emittinglayer/hole-blocking layer/electron-transporting layer/cathode orsubstrate/anode/hole-transporting layer/light-emittinglayer/electron-transporting layer/anode. It is likely that thedeterioration of the shape of thin film is attributable tocrystallization (or cohesion) of thin organic amorphous films caused bygeneration of heat during driving of the element and poor heatresistance is due to low glass transition temperature (Tg) of thematerial in use.

It is described in non-patent literature 2 that a carbazole compound(CBP) or a triazole compound (TAZ) is used in the light-emitting layerand a phenanthroline derivative (HB-1) is used in the hole-blockinglayer. Because of their high symmetry and low molecular weight, thesecompounds readily undergo crystallization or cohesion and sufferdeterioration of the shape of thin film. Besides, theircrystallizability is too high to allow observation of their Tg. Suchinstability of the shape of thin film of the light-emitting layeradversely affects the performance of an element, for example, byshortening the driving life and lowering the heat resistance. For thereasons described above, a difficult problem facing phosphorescentorganic electroluminescent elements at the present time is their drivingstability.

It is disclosed in the aforementioned patent literature 1 that acompound containing an oxadiazolyl group is used as a host material inan organic EL element comprising a host material and a phosphorescentdopant material in its light-emitting layer. An organic EL elementcomprising a thiazole or pyrazole structure in its organic layers isdisclosed in patent literature 2. An organic EL element comprising aphosphorescent iridium complex and a carbazole compound in itslight-emitting layer is disclosed in patent literature 3. An organic ELelement comprising a carbazole compound (PVK), a compound containing anoxadiazolyl group (PBD) and an iridium complex (Ir(ppy)3) in itslight-emitting layer is disclosed in patent literature 4.Ortho-metalated metal complexes and porphyrin metal complexes areproposed as phosphorescent compounds in patent literature 5. However,they face the aforementioned problem. It is to be noted that patentliterature 2 discloses no organic EL elements utilizing phosphorescence.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Improvement of the driving stability and heat resistance of organic ELelements is an essential requirement when their applications to displaydevices such as flat panel displays and illuminating devices areconsidered. In view of the aforementioned present conditions, an objectof this invention is to provide an organic EL element which performswith high efficiency and high driving stability.

Means to Solve the Problems

This invention relates to an organic electroluminescent elementcomprising an anode, organic layers and a cathode piled one upon anotheron a substrate; at least one of the organic layers is a light-emittinglayer containing a host material and a dopant material and apyrazole-derived compound containing 2-4 pyrazole structures representedby the following formula I in the same molecule is used as said hostmaterial.

A compound represented by the following formula II is preferable as sucha pyrazole-derived compound.

In formulas I and II, Ar₁-Ar₃ are independently hydrogen or substitutedor unsubstituted aromatic hydrocarbon groups and X₁ is a direct bond ora substituted or unsubstituted divalent aromatic hydrocarbon group.

A dopant material preferably contains at least one metal complexselected from phosphorescent ortho-metalated metal complexes andporphyrin metal complexes. Organic metal complexes containing at leastone metal selected from groups 7-11 of the periodic table as theircentral metal are also preferable. Preferred examples of this metal areruthenium, rhodium, palladium, silver, rhenium, osmium, iridium,platinum and gold.

An organic electroluminescent element of this invention advantageouslycomprises a hole-blocking layer between a light-emitting layer and acathod or an electron-transporting layer between a light-emitting layerand a cathode.

An organic electroluminescent element (organic EL element) of thisinvention has at least one organic layer which is a light-emitting layerand this light-emitting layer contains a specified host material and aspecified phosphorescent dopant material, with the host materialconstituting the primary component and the dopant material the secondarycomponent.

The primary component here means the component which accounts for 50 wt% or more of the materials constituting the layer in question while thesecondary component means the rest of the materials. A host material hasan excited triplet level higher in energy than that of a dopantmaterial.

According to this invention, a compound to be incorporated in thelight-emitting layer as a host material is required to form a thin filmof stable shape, have a high glass transition temperature (Tg) andtransport holes and/or electrons efficiently. Further, the compound isrequired to be electrochemically and chemically stable and rarelygenerate impurities during manufacture or use which become traps orquench emitted light. As a compound satisfying these requirements, apyrazole-derived compound having a pyrazole structure represented by theaforementioned formula I is used.

In formula I, Ar₁-Ar₃ are independently hydrogen or substituted orunsubstituted aromatic hydrocarbon groups and at least one of Ar₁-Ar₃ isan aromatic hydrocarbon group.

One of the requirements a host material must satisfy in order to form athin film of stable shape is an adequate molecular weight and thepresence of 2 or more pyrazole structures is desirable to satisfy thisrequirement. A host material is usually made into film by vacuumdeposition and an organic compound having a larger molecular weight thanis necessary requires excessive energy in vacuum deposition anddecomposition occurs in preference to evaporation. For this reason, thenumber of pyrazole structures is preferably 4 or so, more preferably 2.

Preferable pyrazole-derived compounds are those represented by theaforementioned formula II wherein Ar₁-Ar₃ are as defined in formula I.

In formulas I and II, Ar₁-Ar₃ are preferably hydrogen or aromatichydrocarbon groups of 1-3 rings and they may have substituents. Thenumber of such substitutents is preferably in the range of 0-3. Thearomatic hydrocarbon groups include aryl groups of 1-3 rings such asphenyl, naphthyl and anthracenyl groups. These groups may havesubstituents; for example, lower alkyl groups of 1-6 carbon atoms suchas methyl and ethyl and aryl groups of 6-12 carbon atoms such as phenyland methylphenyl. Alkyl groups of 1-3 carbon atoms are more preferableas substituents.

Advantageously, Ar₂ is an aromatic hydrocarbon group and one or both ofAr₁ and Ar₃ are aromatic hydrocarbon groups. More preferably, Ar₁ andAr₂ are aromatic hydrocarbon groups and Ar₃ is hydrogen or an aromatichydrocarbon group. Preferred hydrocarbon groups are phenyl, naphthyl,methylphenyl, methylnaphthyl and phenylphenyl groups. A compoundrepresented by formula II wherein Ar₁ and Ar₂ are phenyl groups, Ar₃ ishydrogen or phenyl group and X₁ is phenylene group is cited as anexample of preferred compounds.

Concretely, preferred hydrocarbon groups include the following; phenyl,2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 2,4-dimethylphenyl,3,4-dimethylphenyl, 2,4,5-trimethylphenyl, 4-ethylphenyl,4-tert-butylphenyl, 1-naphthyl, 2-naphthyl, 1-anthracenyl,2-anthracenyl, 9-anthracenyl and 9-phenanthrenyl. The groups Ar₁-Ar₃ maybe identical with or different from one another.

The group X₁ denotes a direct bond or a substituted or unsubstituteddivalent aromatic hydrocarbon group, preferably a divalent aromatichydrocarbon group of 1-3 rings; concrete examples are 1,4-phenylene,1,3-phenylene, 1,4 -naphthylene, 1,5-naphthylene, 2,6-naphthylene,3,3′-biphenylene, 4,4′-biphenylene and 9,10-anthracenylene. Alkyl groupsof 1-6 carbon atoms are cited for substituents. More preferably, X₁ isphenylene, naphthylene or biphenylene group.

An important requirement for forming a thin film of stable shape issuppression of crystallizability. The crystallizability of organiccompounds is considered to be governed by symmetry of the molecularstructure (planarity), intermolecular interaction of polar groups andthe like. The pyrazole-derived compounds to be used in this inventionare prevented from assuming a planar molecular structure by the presenceof an aromatic group at the position 1, 3 or 4 of the pyrazole ring,that is, their crystallizability is suppressed. Furthermore, arrangingbulky hydrocarbon groups around strongly polar nitrogen atoms producesan effect of suppressing also the intramolecular interaction.

Compounds represented by general formula II are listed in Tables 1-6,but they are not limited to these examples. The groups X₁ and Ar₁-Ar₃ inthe tables correspond to those of general formula II. TABLE 1 No. X₁ Ar₁Ar₂ Ar₃ 101

H 102

H 103

H 105

H 106

H 107

H 108

H 109

H 110

111

TABLE 2 No. X₁ Ar₁ Ar₂ Ar₃ 112

113

114

115

116

117

118

119

H 120

H 121

H 122

H

TABLE 3 No. X₁ Ar₁ Ar₂ Ar₃ 123

H 124

H 125

H 126

H 127

H 128

129

130

131

132

133

TABLE 4 No. X₁ Ar₁ Ar₂ Ar₃ 134

135

136

137

H 138

H 139

H 140

H 141

H 142

H 143

H 144

H

TABLE 5 No. X₁ Ar₁ Ar₂ Ar₃ 145

H 146

147

148

149

150

151

152

153

154

155

H

TABLE 6 No. X₁ Ar₁ Ar₂ Ar₃ 156

H 157

H 158

H 159

H 160

H 161

H 162

H 163

H 164

165

166

TABLE 7 No. X₁ Ar₁ Ar₂ Ar₃ 167

168

169

170

171

172

173

H 174

H 175

H 176

H 177

H

TABLE 8 No. X₁ Ar₁ Ar₂ Ar₃ 178

H 179

H 180

H 181

H 182

183

184

185

186

187

188

TABLE 9 No. X₁ Ar₁ Ar₂ Ar₃ 189

190

191

H 192

H 193

H 194

H 195

H 196

H

TABLE 10 No. X₁ Ar₁ Ar₂ Ar₃ 197

H 198

H 199

H 200

201

202

203

204

TABLE 11 No. X₁ Ar₁ Ar₂ Ar₃ 205

206

207

208

An organic EL element to be obtained according to this inventioncomprises a secondary component, that is, a phosphorescent dopantmaterial in its light-emitting layer. Any of the known phosphorescentmetal complexes described in the aforementioned patent literature andnon-patent literature can be used as a dopant material and thesephosphorescent organic metal complexes preferably contain a metalselected from groups 7-11 of the periodic table at the center. Thismetal is preferably ruthenium, rhodium, palladium, silver, rhenium,osmium, iridium, platinum or gold. Either one kind or two kinds or moreof these dopant materials and metals may be used.

Other preferred phosphorescent dopant materials are phosphorescentortho-metalated metal complexes and porphyrin metal complexes and thesemetal complexes are described in patent literature 5 and elsewhere andare publicly known. Therefore, it is possible to use freely these knownphosphorescent dopant materials.

Preferable organic metal complexes include Ir(bt)2.acac3 containing anoble metal such as Ir at the center (formula A), Ir(ppy)3 (formula B)and PtOEt3 (formula C). These complexes are shown below, but not limitedto the examples.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing illustrating the layered structure of anorganic EL element.

NUMBERING IN THE DRAWING

1 substrate; 2, anode; 3, hole-injecting layer; 4, hole-transportinglayer; 5, light-emitting layer; 6, hole-blocking layer; 7,electron-transporting layer; 8, cathode.

PREFERRED EMBODIMENTS OF THE INVENTION

An organic EL element of this invention will be described with referenceto the drawing. In FIG. 1 is schematically illustrated the cross sectionof a structure generally used for an organic EL element. In FIG. 1, asubstrate is designated as 1, an anode as 2, a hole-injecting layer as3, a hole-transporting layer as 4, a light-emitting layer as 5, ahole-blocking layer as 6, an electron-transporting layer as 7 and acathode as 8. Usually, the hole-injecting layer 3 through theelectron-transporting layer 7 are organic layers and an organic ELelement of this invention contains one layer or more, inclusive of thelight-emitting layer 5, of such organic layers. The organic layerscomprise preferably three layers or more, more preferably five layers ormore, inclusive of the light-emitting layer 5. FIG. 1 shows an exampleof the layered structure and it is possible to add or omit one or morelayers.

The substrate 1 supports an organic EL element and is made from a quartzor glass plate, a metal plate or foil or a plastic film or sheet. Inparticular, transparent sheets of synthetic resins such as polyester,polymethacrylate, polycarbonate and polystyrene are desirable. When asynthetic resin is used for a substrate, it is necessary to take the gasbarrier property of the resin into consideration. There is anundesirable possibility of the air passing through a substrate todegrade an organic EL element when the gas barrier property of thesubstrate is too poor. One of the remedial methods is to provide a densesilicon oxide film on at least one side of the synthetic resin substrateto secure the necessary gas barrier property.

The anode 2 is provided on the substrate 1 and plays a role of injectingholes into the hole-transporting layer. The anode is usually constructedof a metal such as aluminum, gold, silver, nickel, palladium andplatinum, a metal oxide such as oxide of indium and/or tin, a metalhalide such as copper iodide, carbon black and conductive polymers suchas poly(3-methylthiophene), polypyrrole and polyaniline. The anode 2 isusually formed by a technique such as sputtering and vacuum deposition.When fine particles of metal such as silver, copper iodide, carbonblack, conductive metal oxides or conductive polymers are used, theparticles are dispersed in a solution of a binder resin and applied tothe substrate 1 to form the anode 2. Moreover, in the case of aconductive polymer, it is possible to form the anode 2 by forming a thinfilm of the polymer directly on the substrate 1 by electrolyticpolymerization of the corresponding monomer or by coating the substrate1 with the conductive polymer. The anode 2 may also be formed by pilingdifferent materials one upon another. The anode varies in thickness withthe requirement for transparency. Where transparency is needed, it ispreferable to keep the transmittance of visible light usually at 60% ormore, preferably at 80% or more. In this case, the thickness is usually5-1000 nm, preferably 10-500 nm. Where opaqueness is acceptable, theanode 2 may be the same as the substrate 1. It is possible to laminate adifferent conductive material on the aforementioned anode 2.

A practice adopted for the purposes of enhancing the efficiency of holeinjection and improving the adhesive strength of the organic layers as awhole to the anode is to interpose the hole-injecting layer 3 betweenthe hole-transporting layer and the anode 2. The interposition of thehole-injecting layer 3 is effective for lowering the initial drivingvoltage of an element and at the same time suppressing a rise in voltagewhen an element is driven continuously at a constant current density.

The material selected for the hole-injecting layer must satisfy thefollowing requirements: it produces a close contact with the anode; itis capable of forming a uniform thin film; it is thermally stable, thatis, it has a melting point of 300° C. or above and a glass transitiontemperature of 100° C. Still further, the material must have lowionization potential which facilitates hole injection from the anode andshow high hole mobility.

So far, phthalocyanine compounds such as copper phthalocyanine, organiccompounds such as polyaniline and polythiophene, sputtered carbonmembranes and metal oxides such as vanadium oxide, ruthenium oxide andmolybdenum oxide have bee reported as materials capable of attainingthis object. In the case of an anode buffer layer, it is possible toform thin films in the same manner as for the hole-transporting layer.In the case of inorganic materials, it is further possible to use suchmethods as sputtering, electron beam evaporation and plasma CVD. Thethickness of the hole-injecting layer 3 thus formed is usually 3-100 nm,preferably 0.5-50 nm.

The hole-transporting layer 4 is provided on the hole-injecting layer 3.The material selected for the hole-transporting layer must be capable ofinjecting holes from the hole-injecting layer 3 at high efficiency andtransporting the injected holes efficiently. To attain this object, thematerial must satisfy the following requirements: it has low ionizationpotential, it is highly transparent against visible light, it shows highhole mobility, it is highly stable and it rarely generates impuritiesduring manufacture or use that become traps. Still more, as thehole-transporting layer exists in contact with the light-emitting layer5, it must not quench the light from the light-emitting layer nor formexciplexes between the light-emitting layer to lower the efficiency.Besides the aforementioned general requirements, heat resistance isadditionally required where application to vehicular displays isconsidered. Hence, the material preferably has a Tg of 90° C. or above.

The compounds useful for such hole-transporting materials includearomatic diamines containing two tertiary amines whose nitrogen atomsare substituted with two or more aromatic condensed rings, aromaticamines of a starburst structure such as4,4′,4″-tris(1-naphthylphenylamino)triphenylamine, an aromatic amineconsisting of a tetramer of triphenylamine and spiro compounds such as2,2′,7,7′-tetrakis-(diphenylamino)-9,9′-spirobifluorene. These compoundsmay be used singly or as a mixture.

Besides the aforementioned compounds, the materials useful for thehole-transporting layer 4 include polymeric materials such aspolyvinylcarbazole, polyvinyltriphenylamine and polyaryleneethersulfonescontaining tetraphenylbenzidine. When the coating process is used informing the hole-transporting layer, a coating solution is prepared bymixing one kind or more of hole-transporting materials and, ifnecessary, binder resins that do not become traps of holes and additivessuch as improvers of coating properties, the solution is applied to theanode 2 or the hole-injecting layer 3 by a process such as spin coatingand the solution is dried to form the hole-transporting layer 4. Thebinder resins here include polycarbonate, polyarylate and polyester.Addition of a binder resin in a large amount lowers the hole mobilityand it is preferably kept at a low level, usually below 50 wt %.

When the vacuum deposition process is used in forming thehole-transporting layer, the selected hole-transporting material isintroduced to a crucible placed in a vacuum container, the container isevacuated to 1×10⁻⁴ Pa or so by a suitable vacuum pump, the crucible isheated to evaporate the hole-transporting material and thehole-transporting layer 4 is formed on the substrate 1 which is placedopposite the crucible and on which the anode has been formed. Thethickness of the hole-transporting layer 4 is usually 5-300 nm,preferably 10-100 nm. The vacuum deposition process is generally used toform such a thin film uniformly.

The light-emitting layer 5 is provided on the hole-transporting layer 4.The light-emitting layer 5 comprises the aforementioned host materialand phosphorescent dopant material and, on application of an electricalfield between the electrodes, the hole injected from the anode andmigrating through the hole-transporting layer recombine with theelectrons injected from the cathode and migrating through theelectron-transporting layer 7 (or the hole-blocking layer 6) to excitethe light-emitting layer thereby causing intense luminescence. Thelight-emitting layer 5 may contain other components, for example,non-essential host materials and fluorescent colorants to the extentthat they do not damage the performance of this invention.

A host material to be used in the light-emitting layer must show a highefficiency of hole injection from the hole-transporting layer 4 and alsoa high efficiency of electron injection from the electron-transportinglayer 7 (or the hole-blocking layer 6). To achieve this end, the hostmaterial must satisfy the following requirements; its ionizationpotential has an adequate value, it shows high mobility of holes andelectrons, it is electrochemically stable and it rarely generatesimpurities during manufacture or use that becomes traps. Still more, thematerial must not form exciplexes between the neighboringhole-transporting layer 4 or the electron-transporting layer 7 (or thehole-blocking layer 6) to lower the efficiency. Besides theaforementioned general requirements, heat resistance is additionallyrequired where application of an element to vehicular displays isconsidered. Therefore, the material preferably has a Tg of 80° C. orabove.

In the cases where one of the organic metal complexes represented by theaforementioned formulas A-C is used as a dopant material, the content ofthe material in the light-emitting layer is preferably in the range of0.1-30 wt %. A content of less than 0.1 wt % does not contribute toimprovement of the luminous efficiency of an element while a content inexcess of 30 wt % causes quenching of light by a change in theconcentration due to dimerization of molecules of the organic metalcomplex and the like and, as a result, the luminous efficiency drops. Inthe conventional elements utilizing fluorescence (singlet), it is adesirable tendency for an organic metal complex to be in an amountsomewhat larger than that of a fluorescent colorant (dopant) containedin the light-emitting layer. The organic metal complex may be containedpartially or distributed nonuniformly in the direction of film thicknessin the light-emitting layer. The thickness of the light-emitting layer 5is usually 10-200 nm, preferably 20-100 nm.

The light-emitting layer 5 is advantageously formed by the vacuumdeposition process. A host material and a dopant material are introducedtogether to a crucible placed in a vacuum container, the container isevacuated to 1×10⁻⁴ Pa or so by a suitable vacuum pump, the crucible isheated to evaporate both host material and dopant material and bothmaterials are deposited on the hole-transporting layer 4. The rates ofdeposition of the host material and dopant material are monitoredseparately to control the content of the dopant material in the hostmaterial.

The hole-blocking layer 6 is formed on the light-emitting layer 5 sothat the blocking layer contacts the light-emitting layer 5 on thecathode side and it is formed by a compound which is capable of playinga role of inhibiting the holes that are migrating through thehole-transporting layer from reaching the cathode and capable oftransporting the electrons that are injected from the cathode in thedirection of the light-emitting layer efficiently. The propertiesrequired for a material constituting the hole-blocking layer are highelectron mobility and low hole mobility. The hole-blocking layer 6 has afunction of confining holes and electrons in the light-emitting layerthereby improving the luminous efficiency.

The electron-transporting layer 7 is formed from a compound which iscapable of transporting the electrons that are injected from the cathodetowards the hole-blocking layer 6 upon application of an electricalfield between the electrodes. An electron-transporting compound usefulfor the electron-transporting layer 7 is required to show highefficiency of electron injection from the cathode 8 and have a highelectron mobility to enable efficient transportation of the injectedelectrons.

The materials satisfying these requirements include metal complexes suchas 8-hydroxyquinoline aluminum complex (Alq3),10-hydroxybenzo[h]quinoline metal complexes, distyrylbiphenylderivatives, silole derivatives, 3- or 5-hydroxyflavone metal complexes,benzoxazole metal complexes, benzothiazole metal complexes,trisbenzimidazolylbenzene, quinoxaline compounds, phenanthrolinederivatives, 2-t-butyl-9,10-N,N′-dicyanoanthraquinonediimine, n-typehydrogenated amorphous silicon carbide, n-type zinc sulfide and n-typezinc selenide. The thickness of the electron-transporting layer 7 isusually 5-200 nm, preferably 10-100 nm.

The electron-transporting layer 7 is formed on the hole-blocking layer 6by a process such as coating or vacuum deposition as in the formation ofthe hole-transporting layer 4. The vacuum deposition process is normallyused.

The cathode 8 plays a role of injecting electrons to the light-emittinglayer 5. A material useful for the cathode 8 may be the same as theaforementioned material for the anode 2. However, a preferred materialis a metal of low work function such as tin, magnesium, indium, calcium,aluminum and silver and alloys thereof. Concrete examples are alloyelectrodes of low work function such as magnesium-silver alloys,magnesium-indium alloys and aluminum-lithium alloys. Furthermore,insertion of an ultrathin insulating film (0.1-5 nm) of LiF, MgF₂, Li₂Oand the like to the interface of the cathode and theelectron-transporting layer is an effective method for improving theefficiency of an element. The thickness of the cathode 8 is usually thesame as that for the anode 2. To protect a cathode made from a metal oflow work function, the cathode is covered with a layer of a metal ofhigh work function and good stability in the air and this improves thestability of an element. Metals such as aluminum, silver, copper,nickel, chromium, gold and platinum are used for this purpose.

It is possible to obtain an element having a structure which is thereverse of the structure shown in FIG. 1: for example, one element isformed by piling one upon another the cathode 8, the hole-blocking layer6, the light-emitting layer 5, the hole-transporting layer 4 and theanode 2 on the substrate 1 and another element is formed in such amanner as to have a structure of substrate 1/anode8/electron-transporting layer 7/hole-blocking layer 6/light-emittinglayer 5/hole-transporting layer 4/hole-injecting layer 3/anode 2.

EXAMPLES Synthetic Example 1

In a 300-ml four-necked flask were placed 10.9 g (0.27 mole) of sodiumhydroxide, 52.7 g of ethanol and 98.3 g of deionized water. The mixturewas stirred at room temperature for 10 minutes until the sodiumhydroxide dissolved and 26.1 g (0.22 mole) of acetophenone was added.Thereafter, the mixture was cooled by ice water and 14.1 g (0.11 mole)of terephthaldehyde was added. After the addition, the mixture washeated under reflux with stirring for 4 hours. Upon completion of thereaction, the reaction mixture was cooled to room temperature and asolid was collected by filtration. The solid was made into a slurry byethanol and dried under reduced pressure to give 28.5 g of a yellowpowder. This powder was analyzed by thin-layer chromatography (TLCG) tobe a single product. Analysis of this product by mass spectrometryshowed that its molecular weight was 338 and identical with that of thetarget compound calcone. The yield of the isolated product was 80.1%.

Synthetic Example 2

In a 2000-ml four-necked flask were placed 20.1 g (0.36 mole) ofpotassium hydroxide and 1482.7 g of ethanol. The mixture was stirred atroom temperature until the potassium hydroxide dissolved and 32.8 g(0.10 mole) of the calcone obtained in Synthetic Example 1 was added atroom temperature. Further, 40.4 g (0.37 mole) of phenylhydrazine wasadded at room temperature. After the addition, the mixture was heatedunder reflux with stirring for 2.5 hours. Upon completion of thereaction, the reaction mixture was cooled to room temperature and asolid was collected by filtration. The solid was washed with methanoland hexane and dried under reduced pressure to give 42.1 g of a yellowpowder. This powder was confirmed to be a single product by TLCG.Analysis of this product by mass spectrometry showed that its molecularweight was 518 and identical with that of the target pyrazolinecompound. The yield was 83.7%.

Synthetic Example 3

In a 500-ml four-necked flask was placed 354.4 g (4.480 moles) ofpyridine. Then, 57.6 g (0.19 mole) of antimony pentachloride was addedslowly in drops so that no vigorous generation of heat occurred. Afterthe dropwise addition, the mixture was allowed to cool to roomtemperature and 25.9 g (0.050 mole) of the pyrazoline compound obtainedin Synthetic Example 2 was added. After this addition, the mixture wasstirred at room temperature for 3.5 hours. Upon completion of thereaction, a solid was recovered by filtration. The solid was washed withethanol to give 42.1 g of a pale brown powder. The powder wasrecrystallized twice from methylene chloride to give 18.3 g of whitecrystals. Analysis of the crystals by TLCG confirmed the formation of asingle product and analysis of the product by mass spectrometry showedthat its molecular weight was 514 and identical with that of the targetcompound 5,5′-(1,4-phenylene)bis[1,3-diphenyl-1H-pyrazole] (hereinafterreferred to as PBP). The melting point was 246.8° C., the yield was71.4% and the compound PBP corresponds to compound No. 101 in Table 1.

The results of the infrared analysis of PBP are shown below.

IR (KBr) 3061, 3047, 1595, 1499, 1487, 1460, 1427, 1358, 1215, 1178,1082, 1066, 972, 954, 847, 795, 773, 764, 696, 602

The sequence of reactions in Synthetic Examples 1-3 are shown below.

Synthetic Example 4

The same procedure as adopted in Synthetic examples 1-3 was followedwith the exception of using 11.0 g (0.28 mole) of sodium hydroxide, 54.0g of ethanol, 100.1 g of deionized water, 37.0 g (0.22 mole) of1-acetonaphthone in place of acetophenone and 14.0 g (0.10 mole) ofterephthaldehyde to synthesize5,5′-(1,4-phenylene)bis[1-phenyl-3-(1-naphthyl)-1H-pyrazole](hereinafter referred to as PBNP). The purity of PBNP was 100% (ratio ofarea in HPLC), the mass obtained in mass spectrometry was 614 and themelting point was 230.2° C. The compound PBNP corresponds to compoundNo. 104.

The results of the infrared analysis of PBNP are shown below.

IR (KBr) 3047, 1595, 1514, 1501, 1431, 1381, 1360, 1321, 1032, 972, 937,912, 849, 814, 802, 773, 739, 692, 660

The sequence of reactions in Synthetic Example 4 is shown below.

Synthetic Example 5

In a 300-ml four-necked flask were placed 94.6 g (0.25 mole) of sodiumbis(methoxyethoxy)aluminum hydride and 72.2 g of THF. The mixture wasstirred at 5° C. or below and 24.2 g (0.28 mole) of morpholine was addedin drops at 5° C. or below. After the dropwise addition, stirring wascontinued at 5° C. or below for 20 minutes to prepare a reagentsolution.

Next, 27.1 g (0.10 mole) of dimethyl 4,4′-biphenyldicarboxylate and133.4 g of THF were introduced to a 2000-ml four-necked reaction vessel.To this solution was added the reagent solution prepared above in dropsat 5° C. or below. After the dropwise addition, stirring was continuedat 15° C. for 4.5 hours. Upon completion of the reaction, extraction wasperformed by adding in drops 622.2 g of methylene chloride and 411.2 gof water. The methylene chloride layer obtained was concentrated untildryness under reduced pressure. The residue was dissolved in acetone byadding 303.2 g of acetone in drops and then 1600.3 g of water was addedin drops at 20° C. Thereafter, the mixture was stirred day and night at5C and a solid was recovered by filtration. The solid thus obtained waswashed with acetone and dried under reduced pressure to give 18.1 g of4,4′-biphenyidicarboxaldehyde.

Using this 4,4′-biphenyldicarboxaldehyde as a starting material, thesame procedure in Synthetic Examples 1, 2 and 3 was followed tosynthesize 4,4′-bis(1,3-diphenyl-5-pyrazolyl)biphenyl (hereinafterreferred to as BPPP). The purity was 99.6% (area ratio in HPLC), themass obtained in mass spectrometry was 590 and the melting point was241.0° C. The compound BPPP corresponds to compound No. 119.

The results of the infrared analysis of BPPP are shown below.

IR (KBr) 3061, 3049, 3028, 1595, 1497, 1456, 1425, 1398, 1362, 1213,1182, 1069, 1005, 970, 955, 829, 806, 766, 692, 681, 600

The reaction sequence in Synthetic Example 5 is shown below.

Synthetic Example 6

In a 300-ml four-necked flask were placed 94.6 g (0.25 mole) of sodiumbis(methoxyethoxy)aluminum hydride and 72.2 g of THF. The mixture wasstirred at 5° C. or below and 24.2 g (0.28 mole) of morpholine was addedin drops at 5° C. or below. After the dropwise addition, the mixture wasstirred continuously at 5° C. or below for 20 minutes to prepare areagent solution.

Thereafter, the procedure in Synthetic Example 5 was followed with theexception of substituting dimethyl 2,6-naphthalenedicarboxylate fordimethyl 4,4′-biphenyldicarboxylate and using 12.2 g of THF tosynthesize 2,6-bis(1,3-diphenyl-5-pyrazolyl)naphthalene (hereinafterreferred to as BPN). The purity was 99.38% (area ratio in HPLC), themass obtained in mass spectrometry was 564 and the melting point was251.7° C. The compound BPN corresponds to compound No. 173.

The results of the infrared analysis of BPN are shown below.

IR (KBr) 3063, 3047, 1597, 1547, 1497, 1456, 1437, 1416, 1364, 1327,1219, 1161, 1074, 988, 957, 895, 862, 818, 806, 766, 694, 677, 662

The reaction sequence in Synthetic Example 6 is shown below.

Synthetic Example 7

In 200-ml four-necked flask were placed 29.4 g (0.15 mole) ofdeoxybenzoin, 10.0 g (0.07 mole) of terephthaldehyde and 77.9 g ofbenzene. The mixture was stirred at room temperature and 2.6 g (0.03mole) of piperidine was added at room temperature. After the dropwiseaddition, the mixture was heated under reflux with stirring for 18 hourswhile removing water from an ester tube. Upon completion of thereaction, the reaction mixture was cooled to room temperature and asolid was recovered by filtration. The crystals obtained were washedwith benzene and dried under reduced pressure to give 28.8 g of3,3′-(1,4-phenylene)bis(1,2-diphenyl-2-propen-1-one).

Then, 20.0 g (0.04 mole) of the3,3′-(1,4-phenylene)bis(1,2-diphenyl-2-propen-1-one) obtained above,497.2 g of ethylene glycol and 35.5 g (0.33 mole) of phenylhydrazinewere introduced to a 1000-ml four-necked flask at room temperature.After the addition, the mixture was heated under reflux with stirringfor 1.5 hours. Upon completion of the reaction, the reaction mixture wascooled to room temperature and a solid was recovered by filtration. Thecrystals obtained were dissolved in methylene chloride and the insolublematters were filtered off. The filtrate was concentrated to drynessunder reduced pressure. To the residue was added in drops 762.3 g ofmethanol and the mixture was heated with stirring for 30 minutes. Afterthe stirring was over, the mixture was cooled to room temperature and asolid was recovered by filtration. The crystals obtained were washedwith methanol and dried under reduced pressure to give 5.6 g of5,5′-(1,4-phenylene)bis(1,3,4-triphenyl-2-pyrazoline).

Next, 93.1 g (1.18 moles) of pyridine was introduced to a 200-mlfour-necked flask. Thereafter, 17.5 g (0.06 mole) of antimonypentachloride was added in drops over 10 minutes so that vigorousgeneration of heat did not occur. After the dropwise addition, themixture was allowed to cool to room temperature and 10.4 g (0.015 mole)of the 5,5′-(1,4-phenylene)bis(1,3,4-triphenyl-2-pyrazoline) obtainedabove was added. After the addition, the mixture was stirred at roomtemperature for 18 hours. Upon completion of the reaction, a solid wasrecovered by filtration. The solid was washed with ethanol and driedunder reduced pressure to give 4.9 g of white crystals. The crystalswere dissolved in methylene chloride and the insoluble matters werefiltered off. The filtrate was concentrated to dryness under reducedpressure. The residue was recrystallized from methylene chloride to give2.4 g of purified white crystals of5,5′-(1,4-phenylene)bis(1,3,4-triphenyl-1H-pyrazole) (hereinafterreferred to as 4-Ph-PBP). The purity (area ratio in HPLC) was 100%, themass obtained in mass spectrometry was 666, and the melting point was312.3° C. The compound 4-Ph-PBP corresponds to compound No. 110.

The results of the infrared analysis of 4-Ph-PBP are shown below.

IR (KBr) 3051, 3034, 1597, 1497, 1452, 1431, 1358, 1323, 1182, 1138,1074, 1026, 968, 908, 854, 775, 760, 752, 700, 658

The sequence of reactions in Synthetic Example 7 is shown below.

Supplementary Example 1

To examine the heat resistance of the candidate compounds for theprimary component (host material) of the light-emitting layer, the glasstransition temperature (Tg) of these compounds was determined bydifferential scanning calorimetry (DSC). The compounds are known hostmaterials and abbreviated as follows; TAZ for3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole, CBP for4,4′-N,N′-dicarbazolediphenyl, BCP for2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and OXD-7 for1,3-bis[(4-t-butylphenyl)-1,3,4-oxadiazolyl]phenylene. The results areshown in Table 12. TABLE 12 Glass transition Host material temperature(Tg) (° C.) PBP 86 PBNP 98 BPPP 106 BPN 96 4-Ph-PBP —¹⁾ TAZ —¹⁾ CBP —¹⁾BCP —¹⁾ OXD-7 —¹⁾¹⁾Not observed

Supplementary Example 2

The stability of the thin films of these host materials was evaluated asfollows. The host material alone was vacuum-deposited on a glasssubstrate to a film thickness of 100 nm. The vacuum-deposited film wasthen stored in an atmosphere where the temperature was kept at 25° C.and the humidity at 30% and the film was visually observed to see howmany days would elapse until it starts to crystallize. The results areshown in Table 13. TABLE 13 Number of days to start of Host materialcrystallization PBP 16 days PBNP 64 days BPPP 55 days BPN 62 days4-Ph-PBP 38 days TAZ  4 days CBP  6 days BCP  5 days OXD-7  3 days

Example 1

An organic EL element having the layered structure shown in FIG. 1 lessthe hole-injecting layer 3 and the hole-blocking layer 6 was prepared asfollows.

The anode 2 was formed by patterning a 2 mm-wide stripe of a transparentconductive ITO film (available from GEOMATEC Co., Ltd.) on the glasssubstrate 1, submitted successively to cleaning with pure water,ultrasonic cleaning with acetone and ultrasonic cleaning with isopropylalcohol, dried by nitrogen blowing, submitted finally toultraviolet/ozone cleaning and set up in an apparatus for vacuumdeposition.

The apparatus was exhausted preliminarily by an oil rotary pump and thenexhausted by an oil diffusion pump equipped with a liquid nitrogen trapuntil the degree of vacuum in the apparatus reached (5-9)×10⁻⁴ Pa. Thehole-transporting layer 4 was formed by heating a molybdenum boatcontaining 4,4′-bis[N,N′-(3-tolyl)amino]-3,3′-dimethylbiphenyl (HMTPD)and depositing the compound to a film thickness of 60 nm. On this layerwas formed the light-emitting layer 5 by co-depositing PBP as theprimary component and tris(2-phenylpyridine)iridium complex (Ir(ppy)3)as the phosphorescent organic metal complex from different evaporationsources to a film thickness of 25 nm. The concentration of Ir(ppy)3 atthis point was 7 wt %. Thereafter, a film of Alq3 with a thickness of 50nm was formed as the electron-transporting layer 7.

The element having the organic layers was taken out of the apparatusinto the air, attached to a mask for vapor deposition of the cathode ora shadow mask in the form of a 2 mm-wide stripe so that the stripe metat right angles with the ITO stripe of the anode 2 and set up in anothervacuum apparatus. After exhausting the apparatus as in the formation ofthe organic layers, lithium fluoride (LiF) as the electron-injectinglayer was deposited on the electron-transporting layer to a thickness of0.5 nm and aluminum as the cathode was deposited to a thickness of 170nm.

The organic electroluminescent element thus obtained was connected to anexternal power source and, when direct current voltage was applied,showed the light-emitting characteristics shown in Table 14. In Table14, the luminous efficiency is a value at 1000 cd/m², luminance/currentmeans the slope of luminance-current density characteristics and thevoltage is a value at 1000 cd/m². The maximum wavelength of the spectrumobserved in emission of light from the element is 515 nm and thisconfirms that Ir(ppy)3 emits light.

Example 2

An organic EL element was prepared as in Example 1 with the exception ofusing PBNP as the primary component of the light-emitting layer 5.Emission of light from Ir(ppy)3 was also confirmed for this organic ELelement.

Example 3

An organic EL element was prepared as in Example 1 with the exception ofusing BPPP as the primary component of the light-emitting layer 5. Thecharacteristics of this element are shown in Table 14.

Example 4

An organic EL element was prepared as in Example 1 with the exception ofusing BPN as the primary component of the light-emitting layer 5.Emission of light from Ir(ppy)3 was also confirmed for this organic ELelement.

Example 5

An organic EL element was prepared as in Example 1 with the exception ofusing 4-Ph-PBP as the primary component of the light-emitting layer 5.The characteristics of this element are shown in Table 14.

Comparative Example 1

An organic EL element was prepared as in Example 1 with the exception ofusing TAZ as the primary component of the light-emitting layer 5. Thecharacteristics of this element are shown in Table 14. TABLE 14 Luminousefficiency Luminance/current Voltage (V) (lm/W) (cd/A) @1000 cd/m2 @1000cd/m2 Example 1 41.03 12.2 10.30 Example 3 38.30 11.0 10.94 Example 528.06 12.3 7.25 Comp. Ex. 1 26.11 13.4 7.13

INDUSTRIAL APPLICABILITY

This invention provides a phosphorescent organic electroluminescentelement with improved luminous efficiency, driving stability and heatresistance applicable to display devices such as flat panel displays andilluminating devices.

1. (canceled)
 2. An organic electroluminescent element comprising ananode, organic layers and a cathode piled one upon another on asubstrate wherein at least one of the organic layers is a light-emittinglayer containing a host material and a dopant material and apyrazole-derived compound represented by the following formula II isused as said host material:

wherein, Ar1-Ar3 are independently hydrogen or substituted orunsubstituted aromatic hydrocarbon groups, at least one of Ar1-Ar3 is agroup other than hydrogen and X1 is a direct bond or a substituted orunsubstituted divalent aromatic hydrocarbon group.
 3. An organicelectroluminescent element as described in claim 2 wherein Ar1 and Ar2are aromatic hydrocarbon groups and Ar3 is hydrogen or an aromatichydrocarbon group in the compound represented by formula II.
 4. Anorganic electroluminescent element as described in claim 2 or 3 whereinAr1 and Ar2 are phenyl groups, Ar3 is hydrogen or phenyl group and X1 isphenylene group in the compound represented by formula II.
 5. An organicelectroluminescent element as described in claims 2 or 3 wherein thedopant material comprises at least one metal complex selected fromphosphorescent ortho-metalated metal complexes and porphyrin metalcomplexes.
 6. An organic electroluminescent element as described inclaim 5 wherein the metal complex comprises at least one metal selectedfrom ruthenium, rhodium, palladium, silver, rhenium, osmium, iridium,platinum and gold at its center.
 7. An organic electroluminescentelement as described in claims 2 or 3 wherein a hole-blocking layer oran electron-transporting layer or both are disposed between thelight-emitting layer and the cathode.